We use the outline of the proof of Theorem 10.2.5 of [1]}. Without loss of generality we may assume \(d_1=1\) .
For \(x,y\in D_r(a)\)  we have
\(|u(x)-u(y)|^p\le (2r)^p\int ^1_0 |u_x(tx+(1-t)y)|^pdt\nonumber \) 
